Transition temperatures and structural correlations for cellulose triesters

Six homologs of the cellulose triester series were found to exhibit, in general, three second-order transitions as determined dilatometrically. A parallel study of torsional modulus versus temperature revealed one glass–rubber transition, which allowed molecular assignment of one of the above. The dependence of the remaining two transition temperatures on length of the ester group permits some speculation with regard to their origin. The glass–rubber and secondary transitions exhibit an inversion for cellulose trivalerate and higher homologs, the two secondary transitions occurring at temperatures higher than the glass–rubber transition. A possible explanation for this anomaly in terms of side-chain crystallization is discussed.     

Simulations of time-dependent fluorescence in nano-confined solvents

The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10-20 angstroms). In previous work [J. Chem. Phys. 118, 6618 (2002)] it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescence measurements of confined solvent systems.     

Diffusion-influenced reversible geminate recombination in one dimension. III. Field effect on the excited-state reaction

We obtain exact analytic solutions of the diffusion-influenced excited-state reversible geminate recombination reaction, A* + B<-->(AB)*, with two different lifetimes and quenching under the influence of a constant external field in one dimension. These fundamental solutions generalize two previous results [Kim et al., J. Chem. Phys. 111, 3791 (1999); 114, 3905 (2001)] and provide us with the insight necessary to analyze their specific relations and asymptotic kinetic transition behaviors. We find that the number of kinetic transitions can be changed due to interplay between the field strength and lifetimes. Unlike the previous works, the number of lifetime dependent transitions is found to be one or zero. On the other hand, the number of the field dependent transitions becomes two, one, or zero. We find a new pattern of kinetic transition e(t)-->t(-1/2)-->e(t) when there is only one field dependent transition.     

Absorption and fluorescence spectroscopy of growing ZnO quantum dots: size and band gap correlation and evidence of mobile trap states

ZnO nanoparticles constitute a convenient model system for fundamental studies with many possible technical applications in, for example, sensors and the field of catalysis and optoelectronics. A large set of ZnO quantum dots in the size range 2.5-7 nm have been synthesized and analyzed in detail. Time resolved in situ UV-vis absorption measurements were used to monitor the growth of these particles in solution by correlating the optical band gap to particle size given from X-ray diffraction (XRD) measurements. The particles formed were isotropic in shape, but small initial deviations gave indications of a transition from thermodynamic to kinetically controlled growth for particles around 4 nm in diameter. On the basis of this, the behavior and mechanisms for the particle growth are discussed. The fluorescence dependence on particle size was investigated by combining fluorescence and UV-vis measurements on growing particles. This revealed that the positions of the fluorescence trap states are mobile toward the conduction- and valence band. A broadening of the trap states was also found, and a surface dependent mechanism of the trap state shift and broadening is proposed.     

Adhesive and mechanical properties of soft nanocomposites: Model studies with blended latex films

We used a unique approach based on contact mechanics to quantify the adhesive and linear viscoelastic properties of latex films approximately 100 μm thick. The latex films were formed from a mixture of two particle types and form stable films consisting of rigid and compliant regions. We used atomic force microscopy to verify that these regions remained well dispersed on the length scale of the original particle size. The properties of the films were determined by ?_h, the volume fraction of the stiffer component. For ?_h < 0.45, the films were quite adhesive, with viscoelastic properties determined by the compliant matrix material. Adhesive interactions between the film and indenter enabled us to oscillate the indenter in the direction normal to the film surface while maintaining a constant contact area, allowing us to determine the frequency dependence of the dynamic moduli of the films. Stiffer films with higher volume fractions of hard particles were characterized by indentation measurements, from which we were able to determine the time dependence of the relaxation modulus of the latex films. All results were consistent with a power‐law form of the relaxation modulus with an exponent of 0.25. The magnitude of the relaxation modulus increased by a factor of 3000 as the volume fraction of hard particles increased from 0 to 0.89. For low values of ?_h, the composition dependence of the film stiffness was similar to the concentration dependence of the viscosity of spherical particle suspensions. A much weaker concentration dependence was observed for the highest values of ?_h, where the properties of the films were dominated by the stiffer component. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3090–3102, 2001     

Excited state electronic structures and dynamics of NOCl: a new potential function set, absorption spectrum, and photodissociation mechanism

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2-7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A' state, though the excitations to the 3 1A' and 3 1A" states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A' state to lower singlet and triplet states are estimated by Fermi's golden rule, and the transitions to the 1 1A' and 3 1A' states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates.     

Two-photon MPI spectroscopy of alkyl iodides

The two-photon resonant multiphoton ionization (MPI) spectra of methyl iodide, methyl iodide-d₃, ethyl, propyl, and butyl iodide are reported in the 49 000-55 000 cm^?1 region. Four separate transitions to excited states labeled Δ, Π, Σ, Π in increasing energy are expected in this range which result from the excitation of an iodine 5pπ electron to the 6s molecular Rydberg orbital. Two-photon spectroscopy with its different selection rules and unique dependence on the laser polarization is shown to significantly advance the understanding of these transitions. In particular, laser polarization studies identify a state which is strongly two-photon allowed but absent in the UV absorption spectrum as the Σ state. Rotational contours indicate a large geometry change takes place in this transition. The two Π states appear strongly in both the one-and two-photon spectrum. Polarization analysis confirms their electronic symmetry assignment in addition to distinguishing vibronic bands arising from nontotally symmetric vibrations. No evidence is found for the Δ state in the multiphoton ionization spectrum, due to either a small two-photon cross section or a low probability of ionization following the initial two-photon transition. Further complications and characteristics of single laser MPI spectroscopy in the study of two-photon absorption in methyl iodide and other fundamental molecules are discussed.     

Temperature dependences of mechanisms responsible for the water-vapor continuum absorption. I. Far wings of allowed lines

It is well known that the water-vapor continuum plays an important role in the radiative balance in the Earth's atmosphere. This was first discovered by Elsasser almost 70 years ago, and since that time there has been a large body of work, both experimental and theoretical, on this topic. It has been experimentally shown that for ambient atmospheric conditions, the continuum absorption scales quadratically with the H(2)O number density and has a strong, negative temperature dependence (T dependence). Over the years, there have been three different theoretical mechanisms postulated: Far wings of allowed transitions, water dimers, and collision-induced absorption. Despite the improvements in experimental data, at present there is no consensus on which mechanism is primarily responsible for the absorption. The first mechanism proposed was the accumulation of the far-wing absorption of the strong allowed transitions. Later, absorption by water dimers was proposed and this mechanism provides a qualitative explanation for the strong, negative T dependence. Recently, some atmospheric modelers have proposed that collision-induced absorption is one of the major contributors. However, based on improvements in the theoretical calculation of accurate far-wing line shapes, ab initio dimer calculations, and theoretical collision-induced absorptions, it is now generally accepted that the dominant mechanism for the absorption in the infrared (IR) windows is that due to the far wings. Whether this is true for other spectral regions is not presently established. Although all these three mechanisms have a negative T dependence, their T dependences will be characterized by individual features. To analyze the characteristics of the latter will enable one to assess their roles with more certainty. In this paper, we present a detailed study of the T dependence of the far-wing absorption mechanism. We will then compare our theoretical calculations with the most recent and accurate experimental data in the IR windows. The results of our calculations are found to agree very well with measurements in the 800-1200 cm(-1) region. We conclude from this work that the T dependence in the IR window region predicted by the far-wing theory is negative and moderately strong. Its pattern is not simple and it could vary significantly as the frequency of interest varies.     

Independence of the brittle–ductile transition from the rubber particle size for impact‐modified poly(vinyl chloride)

A series of acrylic impact modifiers (AIMs) with different particle sizes ranging from 55.2 to 927.0 nm were synthesized by seeded emulsion polymerization, and the effect of the particle size on the brittle–ductile transition of impact‐modified poly(vinyl chloride) (PVC) was investigated. For each AIM, a series of PVC/AIM blends with compositions of 6, 8, 10, 12, and 15 phr AIM in 100 phr PVC were prepared, and the Izod impact strengths of these blends were tested at 23 °C. For AIMs with particle sizes of 55.2, 59.8, 125.2, 243.2, and 341.1 nm, the blends fractured in the brittle mode when the concentration of AIM was lower than 10 phr, whereas the blends showed ductile fracture when the AIM concentration reached 10 phr. It was concluded that the brittle–ductile transition of the PVC/AIM blends was independent of the particle size in the range of 55.2–341.1 nm. When the particle size was greater than 341.1 nm, however, the brittle–ductile transition shifted to a higher AIM concentration with an increase in the particle size. Furthermore, the critical interparticle distance was found not to be the criterion of the brittle–ductile transition for the PVC/AIM blends. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 696–702, 2006     

Density-functional study of model bidisperse ferrocolloids in an external magnetic field

We present phase diagrams of a model bidisperse ferrocolloid consisting of a binary mixture of dipolar hard spheres (DHSs) under the influence of an external magnetic field. The dipole moments of the particles are chosen proportional to the particle volume to mimic real ferrocolloids, and we focus on dipole-dominated systems where isotropic attractive interactions are absent. Our results are based on density-functional theory in the modified mean-field (MMF) approximation. For one-component DHS fluids in external fields, and for corresponding mixtures dominated by one of the components, MMF theory predicts the tricritical point of the transition between an isotropic gas and a ferromagnetic liquid occurring at zero field to be changed into a critical point separating two magnetically ordered phases of different density. The corresponding critical temperature displays a nonmonotonic dependence on the field strength. Completely different behavior is found for the critical temperature related to the demixing phase transitions appearing in strongly asymmetric mixtures [G. M. Range and S. H. L. Klapp, Phys. Rev. E 70, 061407 (2004)]. For such systems, we find a monotonic decrease of the demixing critical temperature with increasing field. The field strength dependence of the critical temperature can therefore be tuned between nonmonotonic and monotonic behaviors just by changing the composition of the mixture--e.g., by adjusting the chemical potentials. This allows us to efficiently control the influence of external magnetic fields on the phase behavior over a large temperature interval.     

Methyl iodide clusters observed in gas phase by infrared cavity ring-down spectroscopy: the CH3 bending mode at 8 microm

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the CH3 bending region at 8 mum by cavity ring-down spectroscopy. The dependence of the spectral features on the mixing ratio of CH3I to He and on the stagnation pressure has allowed us to assign the absorption peaks, with the help of the previous results obtained by matrix-isolation technique [Ito et al., Chem. Phys. Lett. 343, 185 (2001)] and infrared cavity ring-down spectroscopy in the C-H stretching region [Ito et al., Chem. Phys. 286, 337 (2003)]. Ab initio calculations at the MP2 level have been carried out up to tetramer to confirm the assignments. It has been found that the frequency shifts upon clustering (relative to monomer) observed in the bending region are not monotonic, in contrast to those in the C-H stretching region. The observed frequency shifts are discussed in terms of dispersion interaction and its variation upon vibrational excitation.     

Critical behavior of long straight rigid rods on two-dimensional lattices: theory and Monte Carlo simulations

The critical behavior of long straight rigid rods of length k (k-mers) on square and triangular lattices at intermediate density has been studied. A nematic phase, characterized by a big domain of parallel k-mers, was found. This ordered phase is separated from the isotropic state by a continuous transition occurring at an intermediate density theta(c). Two analytical techniques were combined with Monte Carlo simulations to predict the dependence of theta(c) on k, being theta(c)(k) proportional to k(-1). The first involves simple geometrical arguments, while the second is based on entropy considerations. Our analysis allowed us also to determine the minimum value of k (k(min) = 7), which allows the formation of a nematic phase on a triangular lattice.     

Microscopic theory of gelation and elasticity in polymer-particle suspensions

A simplified mode-coupling theory (MCT) of ergodic-nonergodic transitions, in conjunction with an accurate two-component polymer reference interaction site model (PRISM) theory for equilibrium structural correlations, has been systematically applied to investigate gelation, localization, and elasticity of flexible polymer-hard particle suspensions. The particle volume fraction at the fluid-gel transition is predicted to depend exponentially on reduced polymer concentration and size asymmetry ratio at relatively high colloid concentrations. In contrast, at lower particle volume fractions, a power-law dependence on polymer concentration is found with effective exponents and prefactors that depend systematically on the polymer/particle size ratio. Remarkable power-law and near universal scaling behavior is found for the localization length and elastic shear modulus. Multiple experiments for gel boundaries and shear moduli are in good agreement with the no adjustable parameter theory. The one exception is the absolute magnitude of the shear modulus which is strongly overpredicted, apparently due to nonequilibrium dense cluster formation. The simplified MCT-PRISM theory also captures the qualitative aspects of the weak depletion-driven "glass melting" phenomenon at high particle volume fractions. Calculations based on an effective one-component model of structure within a low particle volume fraction framework yield qualitatively different features than the two-component approach and are apparently all in disagreement with experiments. This suggests that volume fraction and size asymmetry dependent many-body screening of polymer-mediated depletion attractions at finite particle concentrations are important.     

Resonance hyper-Raman excitation profiles of a donor-acceptor substituted distyrylbenzene: one-photon and two-photon states

Resonance Raman and resonance hyper-Raman spectra of the "push-pull" conjugated molecule 1-(4'-dihexylaminostyryl)-4-(4"-nitrostyryl)benzene in acetone have been measured at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra), resonant with the first two bands in the linear absorption spectrum. The theory of resonance hyper-Raman scattering intensities is developed and simplified using assumptions appropriate for intramolecular charge-transfer transitions of large molecules in solution. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles, all in absolute intensity units, are quantitatively simulated to probe the structures and the one- and two-photon transition strengths of the two lowest-energy allowed electronic transitions. All four spectroscopic observables are reasonably well reproduced with a single set of excited-state parameters. The two lowest-energy, one-photon allowed electronic transitions have fairly comparable one-photon and two-photon transition strengths, but the higher-energy transition is largely localized on the nitrophenyl group while the lower-energy transition is more delocalized.     

An excited state paired interacting orbital method

A new method for analyzing and visualizing the molecular excited states, named "excited state paired interacting orbital (EPIO)," is proposed. The method is based both on the paired interacting orbital (PIO) proposed by Fujimoto and Fukui [J. Chem. Phys. 60, 572 (1974)] and the natural transition orbital (NTO) by Martin [J. Chem. Phys. 118, 4775 (2003)]. Within the PIO method, orbital interactions between the two fragmented molecules are represented practically only by a few pairs of fragment orbitals. The NTO method is a means of finding a compact orbital representation for the electronic transitions in the excited states. With the method, electronic transitions are expressed by a few particle-hole orbital pairs and a clear picture on the electronic transitions is obtained. EPIO method is designed to have both properties of the preceding two methods: electronic transitions in composite molecular systems can be expressed with a few pairs of EPIOs which are constructed with fragmented molecular orbitals (MOs). Excited state characters, such as charge transfer and local excitations, are analyzed by using EPIOs with their generation probabilities. Thus, the present method gives us clear information on the composition of MOs which play an important role in the molecular excitation processes, e.g., optical processes.     

Theoretical investigation of ground and excited states of the methylene amidogene radical (H(2)CN)

The excited states and the absorption spectrum of the methylene amidogene radical are studied by high-level ab initio calculations. The multireference configuration interaction method was used in combination with different basis sets and basis set extrapolation to compute equilibrium geometries, harmonic frequencies, and excitation energies of the four lowest doublet electronic states of the title species. Potential curves and transition dipole moment functions were determined along the normal mode coordinates of the electronic ground state. These functions were employed to determine vibronic absorption spectra. The intensities of dipole forbidden but vibronically allowed transitions were calculated by explicitly evaluating integrals over the vibrational wave functions and the transition dipole functions of the involved electronic states. By this method the oscillator strengths of the dipole allowed (2)A(1)<--(2)B(2) and the dipole forbidden (2)B(1)<--(2)B(2) bands were computed. It turns out that the dipole forbidden transition is two orders of magnitude weaker than the dipole allowed one. The 0-0 excitation energies are found to be 30 256 cm(-1) for the (2)B(1) state and 34,646 cm(-1) for the (2)A(1) state. From the combined results of the excitation energies and oscillator strengths it is concluded that the experimentally observed peaks must be due to the (2)A(1) state, in contradiction to earlier assignments.     

Dipole strengths of the Qy(0,0) bacteriochlorophyll c transition

The absorption spectra of bacteriochlorophyll (BChl) c solutions in two mixtures of two solvents (acetonitrile with pyridine and dimethylsulfoxide with methanol) exhibiting different refractive indices were measured and deconvoluted into Gaussian components. The refractive index of mixed solvents was changed by the change in the ratio of the volumes of the liquids used in the mixture. Using the Qy(0,0) band half widths and absorption coefficient, based on the simplified formula proposed by Knox, the dipole strengths of the Qy(0,0) BChl c transition for various values of solvent refractive index were calculated and compared with values given by Knox and Spring. For both investigated combinations of two liquids, the dependence of Qy(0,0) transition dipole strength of the BChl c on solvent refractive index was almost linear. The slopes of the lines obtained from the experimental absorption bands were different for two investigated solvent mixtures. More accurate linear dependence and similar slopes of lines for both solvent mixtures were obtained using half widths and absorption coefficients of the Gaussian components of Qy(0,0) transition. It is explained by the superposition of the contributions from other electronic and vibronic transitions of BChl c monomers or possibly also from transitions of the pigments involved in some complexes with solvent molecules in the absorption region investigated. The results show that the formula proposed by Knox can be successfully applied also for BChl c, after elimination of the overlapped contributions from the other transitions, by applying Gaussian analysis to select only contribution from Qy(0,0) pigment transition.     

What's so special about polymers? A little known organizing principle gives rise to an abundance of polymer phase transitions

We argue that if an organizing principle exists it must have to do with the linear connectivity of the monomers since this feature is what distinguishes polymers from all other materials. We then compare a linear polymer threading a pore in a membrane (PTM) with an equal number of unconnected monomers which are also allowed to transit through a pore in a membrane. The crucial difference between the two cases is that the connected monomers are distinguishable from one another by virtue of their location along the polymer chain whereas the disconnected monomers are indistinguishable. Because of this, the disconnected monomers obey the ideal gas laws while the connected monomers undergo a first-order thermodynamic phase transition! Four other phase transitions occurring in isolated linear polymer molecules are known. They are the helix to random coil (HR) transitions in biological polymers, surface adsorption (SA), polymer collapse (C), and a model of polymerization (P). These five kinds of transitions can be coupled to one another resulting in a sizable number of exactly solvable minimal models of phase transitions. There are also five classes of phase transitions in many polymers systems. The coupling of these 10 classes of transitions to each-other results in a plethora of phases. These in turn provide the basis for the many polymer structures observed in the world about us. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2612–2620, 2006     

Semiempirical (ZINDO-PCM) approach to predict the radiative and nonradiative decay rates of a molecule close to metal particles

In this work, we present an extension of a model previously proposed (Andreussi et al. J. Chem. Phys. 2004, 121, 10190) to treat the effect of a metal particle on the optical properties of a molecule in solution (close to such a particle) in the framework of the polarizable continuum model (PCM). This extension concerns the combination of such a model with the semiempirical method Zerner's intermediate neglect of differential overlap (ZINDO), which allows us to treat large size molecular systems, as the ones normally used in the experiments. A refinement of the model is also introduced to take into account the effect of the metal specimen on the absorption process of the molecular system, which affects the probability that a molecule reaches the excited state. Numerical tests are presented to validate the reliability of the ZINDO results with respect to quantum-mechanical DFT methods. Comparisons with experimental results on two different large molecular systems are reported, and the effect of the metal on the absorption process is discussed.     

Two new double Rydberg anions plus access to excited states of neutral Rydberg radicals via anion photoelectron spectroscopy

We have observed and characterized two new double Rydberg anions N6H19- and N7H22- through their anion photoelectron spectra. The vertical detachment energies of these anions were found to be 0.443 and 0.438 eV, respectively. In addition, for three of the seven double Rydberg anions now known, we measured photodetachment transitions not only to the ground electronic states of their corresponding neutral Rydberg radicals but also to their first electronically excited states. In each spectrum, the energy spacing between the resulting peaks provided the ground-to-first electronically excited-state transition energy for the double Rydberg anion's corresponding neutral Rydberg radical. For the radicals, N4H13, N5H16, and N6H19, the spacings were found to be 0.83, 0.70, and 0.67 eV, respectively. These values are in excellent agreement with ground-to-first excited-state transition energies measured in absorption for the same neutral Rydberg radicals by Fuke and co-workers [Eur. Phys. J. D 9, 309 (1999); J. Phys. Chem. A 106, 5242 (2002).] The duplication of this neutral Rydberg property by photodetachment of double Rydberg anions further confirms that double Rydberg anions are indeed the negative ions of their corresponding neutral Rydberg molecules and cluster-like systems.